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<h1>GCC 4.9 Release Series<br />Changes, New Features, and Fixes</h1>


<h2>Caveats</h2>

  <ul>
    <li><p>The mudflap run time checker has been removed.  The mudflap
    options remain, but do nothing.</p></li>

    <li><p>Support for a number of older systems and recently
    unmaintained or untested target ports of GCC has been declared
    obsolete in GCC 4.9.  Unless there is activity to revive them, the
    next release of GCC will have their sources permanently
    <strong>removed</strong>.</p>

    <p>The following ports for individual systems on
    particular architectures have been obsoleted:</p>

    <ul>
	  <li>Solaris 9 (*-*-solaris2.9).  Details can be found in the
	  <a href="https://gcc.gnu.org/ml/gcc-patches/2013-05/msg00728.html">
	      announcement</a>.</li>
    </ul>
    </li>

    <li><p>On AArch64, the singleton vector types <code>int64x1_t</code>,
    <code>uint64x1_t</code> and <code>float64x1_t</code> exported by
    <code>arm_neon.h</code> are defined to be the same as their base types.
    This results in incorrect application of parameter passing rules to
    arguments of types <code>int64x1_t</code> and <code>uint64x1_t</code>,
    with respect to the AAPCS64 ABI specification.  In addition, names of
    C++ functions with parameters of these types (including
    <code>float64x1_t</code>) are not mangled correctly.  The current
    <code>typedef</code> declarations also unintentionally allow implicit
    casting between singleton vector types and their base types.  These
    issues will be resolved in a near future release.  See
    <a href="https://gcc.gnu.org/PR60825">PR60825</a> for more information.
    </p></li>
  </ul>

  <p>
    More information on porting to GCC 4.9 from previous versions
    of GCC can be found in
    the <a href="https://gcc.gnu.org/gcc-4.9/porting_to.html">porting
    guide</a> for this release.
  </p>


<h2>General Optimizer Improvements</h2>

  <ul>
    <li>AddressSanitizer, a fast memory error detector, is now available on ARM.
    </li>
    <li>UndefinedBehaviorSanitizer (ubsan), a fast undefined behavior detector,
        has been added and can be enabled via <code>-fsanitize=undefined</code>.
	Various computations will be instrumented to detect undefined behavior
	at runtime.  UndefinedBehaviorSanitizer is currently available for the C
	and C++ languages.
    </li>
    <li>Link-time optimization (LTO) improvements:
    <ul>
      <li>Type merging was rewritten. The new implementation is significantly faster
      and uses less memory.</li>
      <li>Better partitioning algorithm resulting in less streaming during
	  link time.</li>
      <li>Early removal of virtual methods reduces the size of object files and
	  improves link-time memory usage and compile time.</li>
      <li>Function bodies are now loaded on-demand and released early improving
	  overall memory usage at link time.</li>
      <li>C++ hidden keyed methods can now be optimized out.</li>
      <li>When using a linker plugin, compiling with the <code>-flto</code>
          option now generates slim object files (<code>.o</code>) which only
          contain intermediate language representation for LTO. Use
          <code>-ffat-lto-objects</code> to create files which contain
          additionally the object code.  To generate static libraries suitable
          for LTO processing, use <code>gcc-ar</code> and
         <code>gcc-ranlib</code>; to list symbols from a slim object file use
         <code>gcc-nm</code>. (This requires that <code>ar</code>,
         <code>ranlib</code> and <code>nm</code> have been compiled with
         plugin support.)</li>
    </ul>
    Memory usage building Firefox with debug enabled was reduced from 15GB to
    3.5GB; link time from 1700 seconds to 350 seconds.
    </li>
    <li>Inter-procedural optimization improvements:
    <ul>
      <li>New type inheritance analysis module improving devirtualization.
	  Devirtualization now takes into account anonymous name-spaces and the
	  C++11 <code>final</code> keyword.</li>
      <li>New speculative devirtualization pass (controlled by
	  <code>-fdevirtualize-speculatively</code>.</li>
      <li>Calls that were speculatively made direct are turned back to indirect
	  where direct call is not cheaper.</li>
      <li>Local aliases are introduced for symbols that are known to be
	  semantically equivalent across shared libraries improving dynamic
	  linking times.</li>
    </ul></li>
    <li>Feedback directed optimization improvements:
    <ul>
      <li>Profiling of programs using C++ inline functions is now more reliable.</li>
      <li>New time profiling determines typical order in which functions are
	  executed.</li>
      <li>A new function reordering pass (controlled by
	  <code>-freorder-functions</code>) significantly reduces
	  startup time of large applications.  Until binutils support is
 	  completed, it is effective only with link-time optimization.</li>
      <li>Feedback driven indirect call removal and devirtualization now handle
	  cross-module calls when link-time optimization is enabled.</li>
    </ul></li>
  </ul>

<h2 id="languages">New Languages and Language specific improvements</h2>

  <ul>
    <li>Version 4.0 of the <a href="https://www.openmp.org/specifications/"
	>OpenMP specification</a> is now supported in the C and C++ compilers
	and starting with the 4.9.1 release also in the Fortran compiler.
	The new <code>-fopenmp-simd</code> option can be used to enable OpenMP's
	SIMD directives while ignoring other OpenMP directives. The new <a
	href="https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Optimize-Options.html#index-fsimd-cost-model-908"
	><code>-fsimd-cost-model=</code></a> option permits to tune the
	vectorization cost model for loops annotated with OpenMP and Cilk
	Plus <code>simd</code> directives. <code>-Wopenmp-simd</code> warns when
	the current cost model overrides simd directives set by the user.</li>
    <li>The <code>-Wdate-time</code> option has been added for the C, C++ and
	Fortran compilers, which warns when the <code>__DATE__</code>,
	<code>__TIME__</code> or <code>__TIMESTAMP__</code> macros are used.
	Those macros might prevent bit-wise-identical reproducible
	compilations.</li>
  </ul>

<h3 id="ada">Ada</h3>

  <ul>
    <li>GNAT switched to Ada 2012 instead of Ada 2005 by default.</li>
  </ul>

<h3 id="c-family">C family</h3>

  <ul>
    <li>Support for colorizing diagnostics emitted by GCC has been added.
    The <code><a
    href="https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Language-Independent-Options.html#index-fdiagnostics-color-252"
    >-fdiagnostics-color=auto</a></code> will enable it when
    outputting to terminals, <code>-fdiagnostics-color=always</code>
    unconditionally.  The <code>GCC_COLORS</code> environment variable
    can be used to customize the colors or disable coloring.
    If <code>GCC_COLORS</code> variable is present in the environment,
    the default is <code>-fdiagnostics-color=auto</code>, otherwise
    <code>-fdiagnostics-color=never</code>.<br/>
    Sample diagnostics output:<br/>
    <pre>
    $ g++ -fdiagnostics-color=always -S -Wall test.C
    <b>test.C:</b> In function &lsquo;<b>int foo()</b>&rsquo;:
    <b>test.C:1:14:</b> <span class="boldmagenta">warning:</span> no return statement in function returning non-void [-Wreturn-type]
     int foo () { }
                  <span class="boldlime">^</span>
    <b>test.C:2:46:</b> <span class="boldred">error:</span> template instantiation depth exceeds maximum of 900 (use -ftemplate-depth= to increase the maximum) instantiating &lsquo;<b>struct X&lt;100&gt;</b>&rsquo;
     template &lt;int N&gt; struct X { static const int value = X&lt;N-1&gt;::value; }; template struct X&lt;1000&gt;;
                                                  <span class="boldlime">^</span>
    <b>test.C:2:46:</b>   recursively required from &lsquo;<b>const int X&lt;999&gt;::value</b>&rsquo;
    <b>test.C:2:46:</b>   required from &lsquo;<b>const int X&lt;1000&gt;::value</b>&rsquo;
    <b>test.C:2:88:</b>   required from here

    <b>test.C:2:46:</b> <span class="boldred">error:</span> incomplete type &lsquo;<b>X&lt;100&gt;</b>&rsquo; used in nested name specifier
    </pre></li>

    <li>With the new <a
    href="https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Loop-Specific-Pragmas.html"
    ><code>#pragma GCC ivdep</code></a>, the user can assert that there are no
    loop-carried dependencies which would prevent concurrent execution of
    consecutive iterations using SIMD (single instruction multiple data)
    instructions.</li>

    <li>Support for Cilk Plus has been added and can be enabled
    with the <code>-fcilkplus</code> option.  Cilk Plus
    is an extension to the C and C++ languages to support data and task
    parallelism.  The present implementation follows ABI version 1.2; all
    features but <code>_Cilk_for</code> have been implemented.</li>
  </ul>           

<h3>C</h3>
<ul>
  <li>ISO C11 atomics (the <code>_Atomic</code> type specifier and
  qualifier and the <code>&lt;stdatomic.h&gt;</code> header) are now
  supported.</li>

  <li>ISO C11 generic selections (<code>_Generic</code> keyword) are
  now supported.</li>

  <li>ISO C11 thread-local storage (<code>_Thread_local</code>,
  similar to GNU C <code>__thread</code>) is now supported.</li>

  <li>ISO C11 support is now at a similar level of completeness to ISO
  C99 support: substantially complete modulo bugs, extended
  identifiers (supported except for corner cases
  when <code>-fextended-identifiers</code> is used), floating-point
  issues (mainly but not entirely relating to optional C99 features
  from Annexes F and G) and the optional Annexes K (Bounds-checking
  interfaces) and L (Analyzability).</li>

  <li>A new C extension <code>__auto_type</code> provides a subset of
  the functionality of C++11 <code>auto</code> in GNU C.</li>
</ul>

<h3 id="cxx">C++</h3>
<ul>
  <li>
    The G++ implementation of <a href="../projects/cxx1y.html">C++1y</a> return type deduction for normal
    functions has been updated to conform to
    <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3638.html">N3638</a>, 
    the proposal accepted into the working paper.  Most notably, it adds
    <code>decltype(auto)</code> for getting <code>decltype</code> semantics
    rather than the template argument deduction semantics of plain
    <code>auto</code>:
<blockquote><pre>
int&amp; f();
         auto  i1 = f(); // int
decltype(auto) i2 = f(); // int&amp;
</pre></blockquote>
  </li>
  <li>
    G++ supports <a href="../projects/cxx1y.html">C++1y</a> lambda capture initializers:
<blockquote><pre>
[x = 42]{ ... };
</pre></blockquote>
Actually, they have been accepted since GCC 4.5, but now the compiler doesn't
warn about them with <code>-std=c++1y</code>, and supports parenthesized
and brace-enclosed initializers as well.
  </li>
  <li>
    G++ supports <a href="../projects/cxx1y.html">C++1y</a> variable length
    arrays.  G++ has supported GNU/C99-style VLAs for a long time, but now
    additionally supports initializers and lambda capture by reference.  In
    C++1y mode G++ will complain about VLA uses that are not permitted by
    the draft standard, such as forming a pointer to VLA type or
    applying <code>sizeof</code> to a VLA variable.  Note that it now appears
    that VLAs will not be part of C++14, but will be part of a separate
    document and then perhaps C++17.
<blockquote><pre>
void f(int n) {
  int a[n] = { 1, 2, 3 }; // throws std::bad_array_length if n &lt; 3
  [&amp;a]{ for (int i : a) { cout &lt;&lt; i &lt;&lt; endl; } }();
  &amp;a; // error, taking address of VLA
}
</pre></blockquote>
  </li>
  <li>
    G++ supports the <a href="../projects/cxx1y.html">C++1y</a> [[deprecated]]
    attribute modulo bugs in the underlying [[gnu::deprecated]] attribute.  Classes
    and functions can be marked deprecated and a diagnostic message added:
<blockquote><pre>
class A;
int bar(int n);
#if __cplusplus &gt; 201103
class [[deprecated("A is deprecated in C++14; Use B instead")]] A;
[[deprecated("bar is unsafe; use foo() instead")]]
int bar(int n);

int foo(int n);
class B;
#endif
A aa; // warning: 'A' is deprecated : A is deprecated in C++14; Use B instead
int j = bar(2); // warning: 'int bar(int)' is deprecated : bar is unsafe; use foo() instead
</pre></blockquote>
  </li>
  <li>
    G++ supports <a href="../projects/cxx1y.html">C++1y</a> digit separators.
    Long numeric literals can be subdivided with a single quote ' to enhance readability:
<blockquote><pre>
int i = 1048576;
int j = 1'048'576;
int k = 0x10'0000;
int m = 0'004'000'000;
int n = 0b0001'0000'0000'0000'0000'0000;

double x = 1.602'176'565e-19;
double y = 1.602'176'565e-1'9;
</pre></blockquote>
  </li>
  <li>
    G++ supports <a href="../projects/cxx1y.html">C++1y</a>
    generic (polymorphic) lambdas.
<blockquote><pre>
// a functional object that will increment any type
auto incr = [](auto x) { return x++; };
</pre></blockquote>
  </li>
  <li>
    As a GNU extension, G++ supports explicit template parameter
    syntax for generic lambdas.  This can be combined in the expected
    way with the standard <code>auto</code> syntax.
<blockquote><pre>
// a functional object that will add two like-type objects
auto add = [] &lt;typename T&gt; (T a, T b) { return a + b; };
</pre></blockquote>
  </li>
  <li>
    G++ supports unconstrained <em>generic functions</em> as specified
    by &sect;4.1.2 and &sect;5.1.1 of
    <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3889.pdf">
    N3889: Concepts Lite Specification</a>.  Briefly,
    <code>auto</code> may be used as a type-specifier in a parameter
    declaration of any function declarator in order to introduce an
    implicit function template parameter, akin to generic lambdas.
<blockquote><pre>
// the following two function declarations are equivalent
auto incr(auto x) { return x++; }
template &lt;typename T&gt;
auto incr(T x) { return x++; }
</pre></blockquote>
  </li>
</ul>

  <h4>Runtime Library (libstdc++)</h4>

  <ul>
    <li><a href="https://gcc.gnu.org/onlinedocs/gcc-4.9.2/libstdc++/manual/manual/status.html#status.iso.2011">
       Improved support for C++11</a>, including:
       <ul>
         <li> support for <code>&lt;regex&gt;</code>;</li>
         <li> The associative containers in <code>&lt;map&gt;</code> and
              <code>&lt;set&gt;</code> and the unordered associative containers
              in <code>&lt;unordered_map&gt;</code> and <code>&lt;unordered_set&gt;</code>
              meet the allocator-aware container requirements; </li>
       </ul>
    </li>
    <li><a href="https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2014">
       Improved experimental support for the upcoming ISO C++ standard, C++14</a>,
       including:
      <ul>
        <li> fixing <code>constexpr</code> member functions without <code>const</code>; </li>
        <li> implementation of the <code>std::exchange()</code> utility function; </li>
        <li> addressing tuples by type; </li>
        <li> implemention of <code>std::make_unique</code>; </li>
        <li> implemention of <code>std::shared_lock</code>; </li>
        <li> making <code>std::result_of</code> SFINAE-friendly; </li>
        <li> adding <code>operator()</code> to <code>std::integral_constant</code>; </li>
        <li> adding user-defined literals for standard library types
             <code>std::basic_string</code>, <code>std::chrono::duration</code>,
             and <code>std::complex</code>; </li>
        <li> adding two range overloads to non-modifying sequence oprations
             <code>std::equal</code> and <code>std::mismatch</code>; </li>
        <li> adding IO manipulators for quoted strings; </li>
        <li> adding <code>constexpr</code> members to <code>&lt;utility&gt;</code>,
             <code>&lt;complex&gt;</code>, <code>&lt;chrono&gt;</code>, and some containers; </li>
        <li> adding compile-time <code>std::integer_sequence</code>; </li>
        <li> adding cleaner transformation traits; </li>
        <li> making <code>&lt;functional&gt;</code>s operator functors easier to use
             and more generic; </li>
      </ul>
    </li>
    <li>An implementation of <code>std::experimental::optional</code>.</li>
    <li>An implementation of <code>std::experimental::string_view</code>.</li>
    <li>The non-standard function <code>std::copy_exception</code> has been deprecated
        and will be removed in a future version. <code>std::make_exception_ptr</code>
        should be used instead.
    </li>
  </ul>

<h3 id="fortran">Fortran</h3>
  <ul>
    <li>Compatibility notice:
    <ul>
      <li>Module files: The version of the module files (<code>.mod</code>)
        has been incremented; additionally, module files are now compressed.
        Fortran <code>MODULE</code>s compiled by earlier GCC versions have
        to be recompiled, when they are <code>USE</code>d by files compiled
        with GCC 4.9.  GCC 4.9 is not able to read <code>.mod</code>
        files of earlier GCC versions; attempting to do so gives an error
        message. Note: The ABI of the produced assembler data itself has not
        changed: object files and libraries are fully compatible with older
        versions (except as stated below).</li>
      <li>ABI changes:
      <ul>
        <li>The <a
          href="https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gfortran/Argument-passing-conventions.html"
          >argument passing ABI</a> has changed for scalar dummy
  	  arguments of type <code>INTEGER</code>, <code>REAL</code>,
	  <code>COMPLEX</code> and <code>LOGICAL</code>, which have
	  <em>both</em> the <code>VALUE</code> and the <code>OPTIONAL</code>
	  attributes.</li>
        <li>To support finalization the virtual table associated
          with polymorphic variables has changed.  Code containing
          <code>CLASS</code> should be recompiled, including all files which
          define derived types involved in the type definition used by
          polymorphic variables. (Note: Due to the incremented module version,
          trying to mix old code with new code will usually give an error
          message.)</li>
      </ul></li>
      <li>GNU Fortran no longer deallocates allocatable variables or
        allocatable components of variables declared in the main program. Since
        Fortran 2008, the standard explicitly states that variables declared
        in the Fortran main program automatically have the <code>SAVE</code>
        attribute.</li>
      <li>When opening files, the close-on-exec flag is set if the system
        supports such a feature. This is generally considered good practice
        these days, but if there is a need to pass file descriptors to child
        processes the parent process must now remember to clear the
        close-on-exec flag by calling <code>fcntl()</code>, e.g. via
        <code>ISO_C_BINDING</code>, before executing the child process.</li>
    </ul></li>
    <li>The deprecated command-line option <code>-fno-whole-file</code>
      has been removed. (<code>-fwhole-file</code> is the default since
      GCC 4.6.) <code>-fwhole-file</code>/<code>-fno-whole-file</code>
      continue to be accepted but do not influence the code generation.</li>
    <li>The compiler no longer unconditionally warns
      about <code>DO</code> loops with zero iterations.  This warning is now
      controlled by the <code>-Wzerotrip</code> option, which is implied by
      <code>-Wall</code>.</li>
    <li>The new <code>NO_ARG_CHECK</code> attribute of the <a
      href="https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gfortran/GNU-Fortran-Compiler-Directives.html"
      ><code>!GCC$</code> directive</a> can be used to disable the
      type-kind-rank (TKR) argument check for a dummy argument. The feature
      is similar to ISO/IEC TS 29133:2012's <code>TYPE(*)</code>, except that
      it additionally also disables the rank check. Variables with
      <code>NO_ARG_CHECK</code> have to be dummy arguments and may only be
      used as argument to <code>ISO_C_BINDING</code>'s <code>C_LOC</code>
      and as actual argument to another <code>NO_ARG_CHECK</code> dummy
      argument; also the other constraints of <code>TYPE(*)</code> apply.
      The dummy arguments should be declared as scalar or assumed-size
      variable of type <code>type(*)</code> (recommended) &ndash; or of
      type <code>integer</code>, <code>real</code>, <code>complex</code>
      or <code>logical</code>. With <code>NO_ARG_CHECK</code>, a pointer
      to the data without further type or shape information is passed,
      similar to C's <code>void*</code>. Note that also TS 29113's
      <code>type(*),dimension(..)</code> accepts arguments of any type and
      rank; contrary to <code>NO_ARG_CHECK</code> assumed-rank arguments
      pass an array descriptor which contains the array shape and stride
      of the argument.</li>
    <li><a href="https://gcc.gnu.org/wiki/Fortran2003Status">Fortran 2003</a>:
    <ul>
      <li>Finalization is now supported.  It is currently only done for a
        subset of those situations in which it should occur.</li>
      <li>Experimental support for <em>scalar</em> character components with
        deferred length (i.e. allocatable string length) in derived types has
        been added. (Deferred-length character variables are supported since
        GCC 4.6.)</li>
    </ul></li>
    <li><a href="https://gcc.gnu.org/wiki/Fortran2008Status">Fortran 2008</a>:
    <ul>
      <li>When <code>STOP</code> or <code>ERROR STOP</code> are used to terminate
        the execution and any exception (but inexact) is signaling, a warning is
        printed to <code>ERROR_UNIT</code>, indicating which exceptions are
        signaling. The <code><a
        href="https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gfortran/Debugging-Options.html"
        >-ffpe-summary=</a></code> command-line option can be used to fine-tune
        for which exceptions the warning should be shown.</li>
      <li>Rounding on input (<code>READ</code>) is now handled on systems where
        <code>strtod</code> honours the rounding mode. (For output, rounding is
        supported since GCC 4.5.) Note that for input, the
        <code>compatible</code> rounding mode is handled as <code>nearest</code>
        (i.e., rounding to an even least significant [cf. IEC 60559:1989]
        for a tie, while <code>compatible</code> rounds away from zero in
        that case).</li>
    </ul></li>
  </ul>

<h3 id="go">Go</h3>
  <ul>
    <li>GCC 4.9 provides a complete implementation of the Go 1.2.1
      release.</li>
  </ul>

<!--
<h3>Java (GCJ)</h3>
-->

<h2 id="targets">New Targets and Target Specific Improvements</h2>

<h3 id="aarch64">AArch64</h3>
   <ul>
     <li> The ARMv8-A crypto and CRC instructions are now supported through
       intrinsics. These are enabled when the architecture supports these
       and are available through the <code>-march=armv8-a+crc</code>
       and <code>-march=armv8-a+crypto</code> options.
     </li>
     <li> Initial support for ILP32 has now been added to the
       compiler. This is now available through the command-line option
       <code>-mabi=ilp32</code>. Support for ILP32 is
       considered experimental as the ABI specification is still beta.
     </li>
     <li> Coverage of more of the ISA including the SIMD extensions has
       been added. The Advanced SIMD intrinsics have also been improved.
     </li>
     <li> The new local register allocator (LRA) is now on by default
       for the AArch64 backend.
     </li>
     <li> The REE (Redundant extension elimination) pass has now been enabled
       by default for the AArch64 backend.
     </li>
     <li> Tuning for the Cortex-A53 and Cortex-A57 has been improved.
     </li>
     <li> Initial big.LITTLE tuning support for the combination of Cortex-A57
       and Cortex-A53 was added through the <code>-mcpu=cortex-a57.cortex-a53
       </code> option.
     </li>
     <li> A number of structural changes have been made to both the ARM
       and AArch64 backends to facilitate improved code-generation.
     </li>
     <li> As of GCC 4.9.2 a workaround for the ARM Cortex-A53 erratum
       835769 has been added and can be enabled by giving the
       <code>-mfix-cortex-a53-835769</code> option.
       Alternatively it can be enabled by default by configuring GCC with the
       <code>--enable-fix-cortex-a53-835769</code> option.
     </li>
   </ul>

<h3 id="arc">ARC</h3>
  <ul>
    <li> A port for Synopsys Designware ARC has been contributed by
      Embecosm and Synopsys Inc.</li>
  </ul>

<h3 id="arm">ARM</h3>
     <ul>
      <li> Use of Advanced SIMD (Neon) for 64-bit scalar computations has been
	disabled by default. This was found to generate better code in only
	a small number of cases. It can be turned back on with the
	<code>-mneon-for-64bits</code> option.
      </li>
     <li> Further support for the ARMv8-A architecture, notably implementing
       the restriction around IT blocks in the Thumb32 instruction set has
       been added. The <code>-mrestrict-it</code> option can be used with
       <code>-march=armv7-a</code> or the <code>-march=armv7ve</code> options
       to make code generation fully compatible with the deprecated instructions
       in ARMv8-A.
     </li>
     <li> Support has now been added for the ARMv7ve variant of the
       architecture. This can be used by the <code>-march=armv7ve</code> option.
     </li>
     <li> The ARMv8-A crypto and CRC instructions are now supported through
       intrinsics and are available through the <code>-march=armv8-a+crc</code>
       and <code>mfpu=crypto-neon-fp-armv8</code> options.
     </li>
     <li> LRA is now on by default for the ARM target. This can be turned off
       using the <code>-mno-lra</code> option. This option is a purely
       transitionary command-line option and will be removed in a future
       release. We are interested in any bug reports regarding functional and
       performance regressions with LRA.
     </li>
     <li> A new option <code>-mslow-flash-data</code> to improve performance
       of programs fetching data on slow flash memory has now been
       introduced for the ARMv7-M profile cores.
     </li>
     <li> A new option <code>-mpic-data-is-text-relative</code> for targets
       that allows data segments to be relative to text segments has
       been added. This is on by default for all targets except VxWorks RTP.
     </li>
     <li> A number of infrastructural changes have been made to both the ARM
       and AArch64 backends to facilitate improved code-generation.
     </li>
     <li> GCC now supports Cortex-A12 and the Cortex-R7 through the
       <code>-mcpu=cortex-a12</code> and <code>-mcpu=cortex-r7</code> options.
     </li>
     <li> GCC now has tuning for the Cortex-A57 and Cortex-A53
       through the <code>-mcpu=cortex-a57</code> and <code>-mcpu=cortex-a53
       </code> options.
     </li>
     <li> Initial big.LITTLE tuning support for the combination of Cortex-A57
       and Cortex-A53 was added through the <code>-mcpu=cortex-a57.cortex-a53
       </code> option. Similar support was added for the combination of
       Cortex-A15 and Cortex-A7 through the <code>-mcpu=cortex-a15.cortex-a7
       </code> option.
     </li>
     <li> Further performance optimizations for the Cortex-A15 and the
       Cortex-M4 have been added.
     </li>
     <li>A number of code generation improvements for Thumb2 to reduce code
       size when compiling for the M-profile processors.
     </li>
     </ul>
<h3 id="avr">AVR</h3>
<ul>
  <li>
    A new command-line option <code>-mfract-convert-truncate</code> has been
    added. It allows compiler to use truncation instead of rounding towards
    zero for fractional fixed-point types.
  </li>
</ul>
<h3 id="x86">IA-32/x86-64</h3>
  <ul>
    <li><code>-mfpmath=sse</code> is now implied by <code>-ffast-math</code>
	on all targets where SSE2 is supported.</li>
    <li>Intel AVX-512 support was added to GCC.  That includes inline
      assembly support, new registers and extending existing ones,
      new intrinsics (covered by corresponding testsuite), and basic
      autovectorization.  AVX-512 instructions are available via
      the following GCC switches: AVX-512 foundation instructions:
      <code>-mavx512f</code>, AVX-512 prefetch instructions: <code>-mavx512pf</code>,
      AVX-512 exponential and reciprocal instructions: <code>-mavx512er</code>,
      AVX-512 conflict detection instructions: <code>-mavx512cd</code>.
    </li>
    <li> It is now possible to call x86 intrinsics from select functions in
      a file that are tagged with the corresponding target attribute without
      having to compile the entire file with the <code>-mxxx</code> option.
      This improves the usability of x86 intrinsics and is particularly useful
      when doing <a
      href="https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Function-Multiversioning.html"
      >Function Multiversioning</a>.
    </li>
    <li>GCC now supports the new Intel microarchitecture named Silvermont
      through <code>-march=silvermont</code>.
    </li>
    <li>GCC now supports the new Intel microarchitecture named Broadwell 
      through <code>-march=broadwell</code>.
    </li>
    <li>Optimizing for other Intel microarchitectures have been renamed
      to <code>-march=nehalem</code>, <code>westmere</code>,
      <code>sandybridge</code>, <code>ivybridge</code>,
      <code>haswell</code>, <code>bonnell</code>.
    </li>
    <li><code>-march=generic</code> has been retuned for better support of
      Intel core and AMD Bulldozer architectures.  Performance of AMD K7, K8,
      Intel Pentium-M, and Pentium4 based CPUs is no longer considered important
      for generic.
    </li>
    <li><code>-mtune=intel</code> can now be used to generate code running
      well on the most current Intel processors, which are Haswell
      and Silvermont for GCC 4.9.
    </li>
    <li>Support to encode 32-bit assembly instructions in 16-bit format
      is now available through the <code>-m16</code> command-line option.
    </li>
    <li>Better inlining of <code>memcpy</code> and <code>memset</code> 
	that is aware of value ranges and produces shorter alignment prologues.
    </li>
    <li><code>-mno-accumulate-outgoing-args</code> is now honored when unwind
      information is output.  Argument accumulation is also now turned off
      for portions of programs optimized for size.</li>
    <li>Support for new AMD family 15h processors (Excavator core)
        is now available through the <code>-march=bdver4</code> and
        <code>-mtune=bdver4</code> options.</li>
  </ul>
<h3 id="msp430">MSP430</h3>
  <ul>
    <li>A new command-line option <code>-mcpu=</code> has been added to the MSP430 backend.
    This option is used to specify the ISA to be used.  Accepted values are
    <code>msp430</code> (the default), <code>msp430x</code> and <code>msp430xv2</code>.  The ISA is no longer deduced
    from the <code>-mmcu=</code> option as there are far too many different MCU names.  The
    <code>-mmcu=</code> option is still supported, and this is still used to select linker
    scripts and generate a C preprocessor symbol that will be recognised by the
    <code>msp430.h</code> header file.</li>
  </ul>
<h3 id="nds32">NDS32</h3>
  <ul>
    <li> A new nds32 port supports the 32-bit architecture from Andes
      Technology Corporation.</li>
    <li> The port provides initial support for the V2, V3, V3m
      instruction set architectures.</li>
  </ul>
<h3 id="nios2">Nios II</h3>
  <ul>
    <li> A port for the Altera Nios II has been contributed by
      Mentor Graphics.</li>
  </ul>
<h3 id="powerpc">PowerPC / PowerPC64 / RS6000</h3>
  <ul>
    <li>GCC now supports Power ISA 2.07, which includes support for Hardware
    Transactional Memory (HTM), Quadword atomics and several VMX and VSX
    additions, including Crypto, 64-bit integer, 128-bit integer and
    decimal integer operations.</li>
    <li>Support for the POWER8 processor is now available through the
    <code>-mcpu=power8</code> and <code>-mtune=power8</code> options.</li>
    <li>The libitm library has been modified to add a HTM fastpath that
    automatically uses POWER's HTM hardware instructions when it is
    executing on a HTM enabled processor.</li>
    <li>Support for the new powerpc64le-linux platform has been added.
    It defaults to generating code that conforms to the ELFV2 ABI.</li>
</ul>
<h3>S/390, System z</h3>
  <ul>
    <li>Support for the Transactional Execution Facility included with
      the IBM zEnterprise zEC12 processor has been added.  A set of
      GCC style builtins as well as XLC style builtins are provided.
      The builtins are enabled by default when using
      the <code>-march=zEC12</code> option but can explicitly be
      disabled with <code>-mno-htm</code>.
      Using the GCC builtins also libitm supports hardware
      transactions on S/390.</li>
    <li>The hotpatch features allows to prepare functions for
      hotpatching.  A certain amount of bytes is reserved before the
      function entry label plus a NOP is inserted at its very
      beginning to implement a backward jump when applying a patch.
      The feature can either be enabled per compilation unit via the
      command-line option <code>-mhotpatch</code> or per function using
      the <code>hotpatch</code> attribute.</li>
    <li>The shrink wrap optimization is now supported on S/390 and
      enabled by default.</li>
    <li>A major rework of the routines to determine which registers
      need to be saved and restored in function prologue/epilogue now
      allow to use floating point registers as save slots.  This will
      happen for certain leaf function with <code>-march=z10</code>
      or higher.</li>
    <li>The LRA rtl pass replaces reload by default on S/390.</li>
  </ul>
<h3 id="rx">RX</h3>
  <ul>
    <li> The port now allows to specify the RX100, RX200, and RX600 processors
      with the command-line options <code>-mcpu=rx100</code>,
      <code>-mcpu=rx200</code> and <code>-mcpu=rx600</code>.
    </li>
  </ul>

<h3 id="sh">SH</h3>
  <ul>
    <li>Minor improvements to code generated for integer arithmetic and code
    that involves the T bit.</li>

    <li>Added support for the SH2A <code>clips</code> and <code>clipu</code>
    instructions.  The compiler will now try to utilize them for min/max
    expressions such as <code>max (-128, min (127, x))</code>.</li>

    <li>Added support for the <code>cmp/str</code> instruction through built-in
    functions such as <code>__builtin_strlen</code>.  When not optimizing for
    size, the compiler will now expand calls to e.g. <code>strlen</code> as an
    inlined sequences which utilize the <code>cmp/str</code> instruction.</li>

    <li>Improved code generated around volatile memory loads and stores.</li>

    <li>The option <code>-mcbranchdi</code> has been deprecated.  Specifying it
    will result in a warning and will not influence code generation.</li> 

    <li>The option <code>-mcmpeqdi</code> has been deprecated.  Specifying it
    will result in a warning and will not influence code generation.</li> 
  </ul>

<!--
<h2>Documentation improvements</h2>
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<!--
<h2>Other significant improvements</h2>

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<h2 id="GCC4.9.1">GCC 4.9.1</h2>

<p>This is the <a
href="https://gcc.gnu.org/bugzilla/buglist.cgi?bug_status=RESOLVED&amp;resolution=FIXED&amp;target_milestone=4.9.1">list
of problem reports (PRs)</a> from GCC's bug tracking system that are
known to be fixed in the 4.9.1 release. This list might not be
complete (that is, it is possible that some PRs that have been fixed
are not listed here).</p>

<p>Version 4.0 of the OpenMP specification is supported even in Fortran,
not just C and C++.</p>

<h2 id="GCC4.9.2">GCC 4.9.2</h2>

<p>This is the <a
href="https://gcc.gnu.org/bugzilla/buglist.cgi?bug_status=RESOLVED&amp;resolution=FIXED&amp;target_milestone=4.9.2">list
of problem reports (PRs)</a> from GCC's bug tracking system that are
known to be fixed in the 4.9.2 release. This list might not be
complete (that is, it is possible that some PRs that have been fixed
are not listed here).</p>

<h2 id="GCC4.9.3">GCC 4.9.3</h2>

<p>This is the <a
href="https://gcc.gnu.org/bugzilla/buglist.cgi?bug_status=RESOLVED&amp;resolution=FIXED&amp;target_milestone=4.9.3">list
of problem reports (PRs)</a> from GCC's bug tracking system that are
known to be fixed in the 4.9.3 release. This list might not be
complete (that is, it is possible that some PRs that have been fixed
are not listed here).</p>

<h2 id="GCC4.9.4">GCC 4.9.4</h2>

<p>This is the <a
href="https://gcc.gnu.org/bugzilla/buglist.cgi?bug_status=RESOLVED&amp;resolution=FIXED&amp;target_milestone=4.9.4">list
of problem reports (PRs)</a> from GCC's bug tracking system that are
known to be fixed in the 4.9.4 release. This list might not be
complete (that is, it is possible that some PRs that have been fixed
are not listed here).</p>

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